The present invention relates to rotary power tools and in particular to mechanisms for controlling the speed of a rotary power tool output shaft. U.S. Pat. No. 7,124,839 discloses a multi-speed transmission for a rotary power tool mediated by two sets of planetary gears. The speed of the output shaft is determined by adjusting the axial position of one of the ring gears. In a first axial position, the ring gear cannot rotate, so both planetary gear sets contribute to speed reduction, and the tool operates in low speed mode. In a second axial position, the ring gear is free to rotate and is coupled with the sun gear of the second planetary gear set. As a result, there is no speed reduction by the first set of planetary gears and the tool operates in a high speed mode.
Without support structures to stabilize this ring gear, it would be susceptible to vibration and vibration-associated noise. However support structures tend to introduce friction which generates heat as the ring gear rotates at high speeds. In the '839 patent, the inner surface of a precision ball bearing is provided to support smooth rotation of the ring gear while minimizing friction. A switching ring secures the outer surface of the ball bearing and provides means for adjusting the axial position of the ball bearing and ring gear assembly. A disadvantage of this solution is the high cost of the precision ball bearing used to stabilize the ring gear.